The prior art relating to this invention will be explained by using the example of FIG. 1 where, in order to make an electrical contact with source or drain n.sup.+ regions 82 of an MOS LSI formed on a p type Si substrate, contact holes 85 are formed in an insulating film 81 such as SI0.sub.2, and then tungsten (W) electrodes are attached to the surface of n.sup.+ regions 82 located inside contact holes 85.
As a method for manufacturing the W electrodes, the method that by introducing WF.sub.6 and H.sub.2 gases into a reaction chamber, W electrodes are formed on surface 84 of n.sup.+ region 82 according to the following equation: EQU WF.sub.6 +3H.sub.2 .fwdarw.W+6HF . . . (1),
has been examined.
The reaction shown as (1) is thought to proceed in such a manner that deposited tungsten works as a catalyst, and the WF.sub.6 is reduced by H.sub.2 as it is adsorbed by the W. Therefore, this reaction is thought to proceed swiftly only in the case in which W has been deposited thicker than some particular thickness and the surface of the metal is kept pure. If the surface of W is not pure, the substrate must be heated to a temperature higher than 600.degree. C. in order for the reaction (1) to proceed. However, when the temperature of the substrate is increased, not only is the quality of the deposited W film reduced because the size of the crystal grain becomes large, but the chance of the formation of an undesired film is also increased as a result of a reaction between tungsten and impurities.
In addition, in the case of the example of FIG. 1, it is necessary to deposit the W film only on the surface of the Si located inside contact holes 85 without depositing it on insulating film 81. For that process to occur, the surface of n.sup.+ region 82 is required to be pure semiconductor. The reason is as follows.
The initial deposition of the W is thought to occur selectively only on the semiconductor surface according to the following reduction reaction EQU 2WF.sub.6 +3Si.fwdarw.2W+3SiF.sub.4 . . . (2)
after WF.sub.6 is adsorbed by Si surface 84 of n.sup.+ region 82. Therefore, if some other compounds are formed on the surface of n.sup.+ region and thus the pure semiconductor surface is not preserved, the reaction (2) will not proceed swiftly and, in other words, the selective deposition will not occur.
In order to keep the surface pure, it is important not to allow impurities, especially H.sub.2 O, O.sub.2, and hydrocarbons such as CH.sub.4 to exist in the reaction chamber. When the structure shown in FIG. 1 is adapted to an ultra high integration of LSI, the concentration of these impurities must be suppressed less than several ppb.
However, there exist at the present time no apparatuses which attain such a high purity atmosphere in the reaction chamber. Namely, in the prior art W film forming apparatus, organic materials which are corrosion-resistant to WF.sub.6 are used for the piping system, and metal materials such as stainless steel are used for the reaction chamber. As a result, a large amount of impurities from several ppm to several hundreds ppm, mainly consisting of moisture and hydrocarbons are released from the organic materials. These impurity gases are mixed with source gases, which causes various problems as shown by the following examples.
1) When the moisture adheres to the metal materials of the reaction chamber, the following reaction occurs and products such as a gas cause a secondary contamination, EQU M+H.sub.2 O.fwdarw.MO+H.sub.2. PA1 2) When the source gas WF.sub.6 reacts with moisture, HF is generated according to the reaction, EQU WF.sub.6 +H.sub.2 O.fwdarw.WOF.sub.4 +2HF. PA1 a) Selective deposition between the insulating film 81 and the surface 84 of n.sup.+ region 82 will not be obtained. PA1 b) The reactions shown in (1) and (2) will not proceed at low temperature. In order for the reactions to proceed, the substrate needs to be maintained at a temperature higher than 600.degree. C. However, in this case, the grain size of crystal in the W film gets large, and the film quality therefore deteriorates.
Here, M represents the metal material used for the reaction chamber.
The surface of the reaction chamber is corroded by HF, and undesirable gases such as oxygen are produced. This case also causes a secondary contamination.
These types of secondary contamination do harm to the semiconductor devices. For example, if H.sub.2 O and O.sub.2 is adsorbed by the surface of the n.sup.+ region 82, the surface 84 will be oxidized. In the case where the substrate is silicon, SiO.sub.2 will be formed on the surface of the n.sup.+ region. Consequently,
If H.sub.2 O or O.sub.2 is adsorbed by the insulating film, the site where H.sub.2 O or O.sub.2 is adsorbed serves as a source of nucleation in a deposition process, and the W film results even on the insulating film. Namely, the selectivity of deposition between n.sup.+ region 82 and insulating film 81 will be lowered.
Furthermore, when WF.sub.6 gas of low grade is used, which gas, for example, usually includes more than 10 ppm of moisture, WF.sub.6 reacts with H.sub.2 O, generating HF which is extremely reactive, and HF thus generated corrodes not only metal but also polyimides and CTFE which are used for, for example, the seat of a valve and the conductance controller of a regulator. As a result, the purity of the source gas is decreased still more.
For these reasons, the W films manufactured by using the prior art technologies are of poor quality, and the specific contact resistivity value of the film is as high as 10.sup.-4 to 10.sup.-5 ohm/cm2.
In addition, the W film deposits on the insulating film as well as on the surface of Si. Thus, another aim to form the W film only on the Si surface located inside the contact holes has not been attained.